When two user equipment terminals (e.g., mobile communication devices) of a cellular network or other telecommunication system communicate with each other, their data path typically goes through the operator network. The data path through the network may include base stations and/or gateways. If the devices are in close proximity with each other, their data path may be routed locally through a local base station. In general, communications between a network node such as a base station and a wireless terminal is known as “wide area network” (“WAN”) or “Cellular communication”.
It is also possible for two user equipment terminals in close proximity to each other to establish a direct link without the need to go through a base station. Telecommunications systems may use or enable device-to-device (“D2D”) communication, in which two or more user equipment terminals directly communicate with one another. In D2D communication, voice and/or data traffic (referred to herein as “communication signals”) from one user equipment terminal to one or more other user equipment terminals may not be communicated through a base station or other network control device of a telecommunication system. Device-to-device (D2D) communication has more recently also become known as “sidelink direct communication” or even “sidelink” communications, and accordingly is sometimes abbreviated as “SLD” or “SL”. As such, device-to-device (D2D), sidelink direct, and sidelink are used interchangeably herein but all have the same meaning.
Various aspects of sidelink direct communications are described in one or more of the following, all of which are incorporated herein by reference in their entirety:    U.S. patent application Ser. No. 14/660,528, filed Mar. 17, 2015;    U.S. patent application Ser. No. 14/660,491, filed Mar. 17, 2015;    U.S. patent application Ser. No. 14/660,559, filed Mar. 17, 2015;    U.S. patent application Ser. No. 14/660,587, filed Mar. 17, 2015;    U.S. patent application Ser. No. 14/660,622, filed Mar. 17, 2015;    U.S. patent application Ser. No. 14/749,898, filed Jun. 25, 2015;    U.S. Provisional Patent application 62/055,114, filed Sep. 25, 2014;    U.S. Provisional Patent application 62/104,365, filed Jan. 16, 2015;
What is needed therefore, and an object of the technology disclosed herein, are apparatus, methods, and techniques for carrying sufficient synchronization information for D2D communications using a D2D synchronization signal.